1. Field of the Invention
The present invention relates to a capacitance type electromagnetic flowmeter and, in particular, to the structure of a joint section between a flow tube and a casing member encasing the same in a capacitance type electromagnetic flowmeter.
2. Description of the Prior Art
Hitherto, a variety of patent applications have been filed regarding electromagnetic flowmeters and, in particular, capacitance type electromagnetic flowmeters, a typical example of which is shown in U.S. Pat. No. 4,539,853.
FIG. 2 is a side view of the prior-art example mentioned above.
Referring to the drawing, a flange section 31a, 3lb is provided at either end of a flow tube 31 formed of a ceramic material. The flow tube 31 is encased in a casing member 32. The outer periphery of each flange section 31a, 3lb is fitted into the casing member 32 and firmly joined thereto by metallic bonding, resin adhesion or the like. A pair of coils 33 and a core 34 for generating a magnetic field in the flow tube are firmly attached to the cylindrical outer peripheral surface of the flow tube 31. Further, a pair of flow electrodes are provided on the outer peripheral surface of the flow tube 31 in such a way as to be positioned parallel to the magnetic flux emitted from the coils 33. Further, a shield electrode (not shown) is provided in such a way as to surround the flow electrodes. Protective rings 6aand 6b are respectively attached to the end surfaces of the flange sections 31a and 3lb of the flow tube 31 via first gaskets 5a and 5b, respectively. The protective rings 6a and 6b serve to protect the flow tube 31 and to set the electric potential of the liquid portion near the protective rings to the ground potential. Second gaskets 7a and 7b are attached to the respective reverse or outer sides of the protective rings 6a and 6b. The gaskets 5a, 5b, 7a and 7b are mainly formed of a highly waterproof sealing material such as polytetrafluoroetylene. The electromagnetic flowmeter of the described structure is interposed between flanges 8a and 8b of a process piping and fastened thereto by bolts 9 so as to be held in a liquid-tight fashion by the process piping.
Due to the provision of an electrode section which is relatively free from the influence of insulating extraneous matter, the capacitance type electromagnetic flowmeter described above has excellent features which are not to be expected from electrode type electromagnetic flowmeters. For example, the main cause of flow liquid leakage is eliminated in the above capacitance type electromagnetic flowmeter. On the other hand, this electromagnetic flowmeter has a problem that its output is liable to fluctuate depending upon the magnitude of vibrations propagated thereto from the outside, for example, piping vibrations. Such a fluctuation in output is caused as follows: when the flow tube is caused to vibrate by external vibrations, a vibration having a specific frequency causes the electromagnetic flowmeter to resonate to generate a distortion between the outer peripheral ceramic surface of the flow tube and the flow electrodes provided on the ceramic surface. This distortion causes a microphone noise to be generated in the electrodes.